A crucial application for radio frequency (RF) transmitters and other beacon type devices is the location of persons in need of assistance, i.e. survivors of plane crashes, shipwrecks or other emergency situations. Rescue devices of this type emit radio signals and/or visual signals which allow a rescue team to converge on the location of the user. Often, these devices are fixed components of aircraft and ships. Alternatively, survival packs, which contain equipment for sending distress signals are often included in survival gear. For example, on military aircraft the pilot is equipped with a radio which automatically emits an RF signal upon the pilot ejecting from the plane. The radio is worn directly on the survival vest of the pilot.
Also, individuals may find themselves in distressed conditions. In many instances, the site of the distress may be in an area that inhibits radio or other communication transmission.